S

Abbreviation for second.

Safety

Power supplies play an essential role in the safety of most electronic systems. They are the interface between the dangerous voltages of the mains supply and the low voltages used in the user/equipment interface. Because of this, many safety standards focus on the power supply as the item to ensure safe operation.

Safety Approved

Certification, recognition or approval by safety agencies such as BSI, TUV,VDE,UL,CSA etc.

Safety Compliance

Complies with the various safety requirements of the product or power supply.

Safety Earth

Safety Extra Low Voltage (SELV)

Internationally recognised safe voltage level which can be touched by a user.

IEC define it as “voltage which does not exceed 50Vac or 120V ripple-free dc between conductors, or between any conductor and earth, in a circuit which is isolated from the supply mains by such means as a safety isolating transformer”. However, different safety standards have different names and values for "safe voltage" and some of the most common standards are as follows :

Secondary Winding

SELV

Semi-regulated Output

A subjective term indicating partial regulation (usually applies to secondary outputs of a Multiple Output Power Supply). Typically with semi-regulated outputs, the Cross Regulation specification is poor (or not specified at all), the Load Regulation specification is poor (for example, if a power supply can provide more than 1A and the specification is +/-5% or worse then this is likely to be a semi-regulated output). Power supplies with semi-regulated outputs often have a Minimum Load requirement on at least one of the outputs. Semi-regulated outputs may well be acceptable for most applications, however, some things to be aware of and consider before using a semi-regulated output:-

If you have a variable load on the power supply then all semi-regulated outputs will have transients on the output at the same frequency as the load variations.

If the load on a semi-reg o/p is removed (or reduced) then the output will increase. On some power supplies, this can be as much as 30% or more!!!

If you use remote sense on the main (usually fully regulated) output, then all semi-regulated outputs could vary by a similar amount (depending on exact design). For example, with a 3V channel 1, if you have 0.5V of remote sense correction then the output of a 12V semi-regulated supply could vary by up to 2V !!

Great care and consideration needs to be paid to ensure that these effects (and possibly others) will not cause any problems. It is also useful to know if the power supply you are considering using has semi-reg outputs as this is often not shown in product datasheets (although by checking the Cross Regulation, Load Regulation or Total Regulation specification, it should be possible to deduce this).

Sense

The connection which allows the power supply control loop to see the voltage present at the load. See also Remote Sensing.

Sense -ve

Sequencing

Series

Connecting two or more power supplies (+ve of PSU 1 to -ve of PSU 2 and so on) to increase the output voltage (which will simply be the sum of all output voltages). This is only possible with isolated outputs and particular care needs to be paid to short circuit currents. It is better to only series connect outputs which have the same or similar output currents. The maximum output current will be the lowest of each of the series connected power supplies. For example. Connecting a 24V / 10A power supply in series with a 12V / 20A power supply will provide 36V / 10A (the sum of 24V and 12V for the output voltage and the lowest of 20A and 10A or the output current). Care needs to be taken when connecting outputs in series to ensure that the output isolation to ground is not exceeded.

Series Regulator

Setting Accuracy

The accuracy to which the outputs of a power supply are set. For example, if a 12V power supply has a setting accuracy of 1% then the output voltage range could be 11.88 – 12.12 V.

Setting Range

The range over which the value of the stabilised output quantity may be adjusted. Usually applies to output voltage. See also Adjustment Range.

Settling Time

The time for a power supply (PSU) to stabilise within specifications after an excursion outside the input/output design parameters.

Shelf Life

The time it is possible to store a power supply under specified conditions and still retain the ability to operate to specification.

Shock Hazard

A potentially dangerous situation in which current can pass through a person or animal.

Short-Circuit

A direct connection that provides a virtually zero resistance path for current.

Short-Circuit Current (SCC)

The initial value of the current obtained from a power supply in a circuit of negligible resistance.

Short-Circuit Protection

A protective feature that limits the output current of a power supply (PSU) to prevent damage to the power supply. Care must be taken to ensure that connecting leads and any PCB tracks are designed to allow for the maximum current possible from the power supply (or supply additional protection circuitry / fusing for protection).

Short-Circuit Test

Shorting the output to ensure that the short circuit current is within its specified limits.

Shunt Regulator

A linear regulator in which the control element (usually a Zener Diode) is in parallel with the load, and in series with an impedance, to achieve constant voltage across the load.

SiC

Siemens (S)

Signal Ground

The common return or reference point for analogue signals.

Silicon Carbide Schottky Diodes

SiC Schottky diodes have essentially no reverse recovery current, and the minimal amount of capacitive current present in turn-off is independent from temperature, forward current, and di/dt. This results in virtually no switching losses for the rectifier and substantially lessens switching losses in the switch. The use of silicon carbide diodes in the pfc circuit (as in TDK-Lambda's NV-Power range of power supplies) can offer around 4% efficiency improvement.

Silicon-Controlled Rectifier (SCR)

A four layer (PNPN) junction device which controls current flow. Conduction is initiated by the application of a gate current. Conduction will continue until the current is reduced to some minimum value.

Schematic Symbol for an SCR

Sine Wave

A wave form of a single frequency alternating current whose displacement is the sine of an angle proportional to time or distance.

Single Point Ground

The one point in a system that connects multiple grounds and returns. Also known as star ground or star point ground.

Snubber / Snubber Network

Soft Start

Soldering

Joining metal surfaces by fusion of a metal alloy such as tin, silver and copper (SnAgCu). It forms a joint of low Resistance. Previously, solders with a high lead content were commonplace but have since been largely replaced by lead-free solders (to comply with theRoHS Directive).

Solid-state

Originally meaning circuits which do not contain vacuum tubes (valves) but has come increasingly to mean circuits with no moving parts.

Solid-state Switch

Source Impedance

Spacings

SPS

Abbreviation for Standby Power Supply.

Stability

The amount of change in an output parameter (with all other factors constant) as a function of time after a specified warm-up period.

Standard Wire Gauge

A standard for sizing wire diameters and for measuring sheet-metal thicknesses. Note that SWG is used mainly in the UK and is different from the thicknesses/diameters associated with AWG numbers. Also called Imperial Standard Wire Gauge.

Stand-off

A mechanical support, either conducting or insulating, used to support a wire, device or PCB away from the mounting surface.

Storage Temperature Range

Supplementary Insulation

An independent Insulation provided in addition to the basic insulation to protect against electric shock in case of mechanical rupture or electrical breakdown of the basic insulation. Used together with Basic Insulation provides Double Insulation.

Surface Mount Device (SMD)

A type of component which is intended to be mounted directly upon the surface of a printed circuit board.

SWG

Switched Mode Power Supply (SMPS)

A device which supplies electrical energy to a load using switching technology. SMPS are used as replacements for linear supplies when higher efficiency, smaller size or lighter weight are required. There are many types of different power supplies including Linear Supplies, Switched Mode Power Supplies, DC-DC converters,Programmable Power Supplies. Lambda is able to supply many different types of power supplies to meet your requirements from only a few watts up to 60kwatts.

Switching Frequency

The rate at which the dc voltage is switched in a converter or power supply (PSU). In power factor correctedPSUs, there will be two switching frequencies, the boost converter switching frequency and the forward converter switching frequency. Additionally, some power supplies may have additional converters with their own switching frequencies (for example buck regulators).

Switching Regulator

A voltage regulator which operates by rapidly switching the current into the load to stabilise the output voltage. They are inherently more efficient (up to 90% and above), smaller and lighter than linear regulators but are most complicated. See Linear Regulation.

Synchronous Rectification

A rectification scheme in a switched mode power supply (PSU) in which a FET or bipolar transistor is substituted for the rectifier diode to improve efficiency.